A digital receiver is an electronic system for digital communications receiving an analog input signal and providing a digital output signal, and includes an amplifier, a filter, and a digitizer in the analog front-end.
In particular, the amplifier increases the input dynamic range of the receiver, thus helpfully providing a good level of the analog output signal. Moreover, the filter limits the noise bandwidth of the receiver, and the digitizer converts the analog input signal into the digital output signal.
The digitizer can be realized using a simple comparator or using a more complex “Analog to Digital Converter,” depending on the input signal characteristics. In particular, a simple comparator can be used for distinguishing two different voltage signal levels. This is the case of a digital receiver having as input voltage signal an ASK [Amplitude Shift Keying] modulation signal, having an amplitude shift between zero and a power supply voltage Vdd, or an OOK [On-Off Keying] modulation signal.
A known digitizer 100 is shown in FIG. 1. It has an input terminal INPUT, which receives a modulated analog input voltage signal Vin, and an output terminal OUTPUT which provides an output signal Vout being a digital conversion of the input voltage signal Vin. The digitizer 100 further includes an R-C filter 102 which extracts the DC component of the analog voltage input signal Vin and provide it as voltage reference signal Vref to a comparator 101.
This simple circuit realizing the digitizer 100 may ensure that the voltage reference signal Vref is set between the voltage signals levels corresponding to a “0” logic transmitted and to “1” logic transmitted. Nevertheless, the limit of this circuit is that it works well so long as the data received is reasonably DC-balanced, or rather when the voltage reference signal Vref is more or less in a symmetric position between the minimum and the maximum of input signal.
In fact, it is clear from the response of the digitizer 100, shown in FIG. 2, that if the analog voltage input signal Vin is not symmetrical in respect with the DC component value, the voltage reference signal Vref may not be a good voltage reference signal for the comparator 101. Moreover, even the noise margin is very low, since a very low analog voltage input signal Vin would be close to the voltage reference signal Vref, causing the comparator be unstable.
In order to address this issue, some other architectures have been proposed. For example, U.S. Pat. No. 5,003,196 describes a Schmitt trigger circuit comprising a detector of maximum and minimum voltage and two comparators, sensing the rising and the falling edges of the input voltage signal Vin and converting them into an output digital signal using a SR (set-reset) flip-flop. Even though advantageous under several aspects, the described Schmitt trigger circuit may not be used in some kind of applications, for instance optical receivers.
The technical application underlying the present approach is that of providing a digitizer, and a method for digitizing, having structural and functional characteristics which allow the digitization of both a high input voltage signal and a low input voltage signal, and to help reduce effects on signal noise when the input voltage signal is null, in this way overcoming the limits which affect the devices realized according to the prior art.